Transistorized horizontal deflection circuits may be made up of a horizontal switching or output transistor, a diode, one or more capacitors and a deflection winding. The output transistor, operating as a switch, is driven by a horizontal rate square wave signal and conducts during a portion of the horizontal trace interval. A diode, connected in parallel with the transistor, conducts during the remainder of the trace interval. A retrace capacitor and the deflection yoke winding are coupled in parallel across the transistor-diode combination. Energy is transferred into and out of the deflection winding via the diode and output transistor during the trace interval and via the retrace capacitor during the retrace interval.
In some television receivers, the collector of the horizontal output transistor is coupled to the B+ power supply through the primary windings of the high voltage transformer. Under certain conditions, the transistor may be subject to abnormally high collector currents, which may damage the transistor. For example, if arcing should occur within the kinescope, the high voltage winding of the high voltage transformer is effectively short-circuited to ground, which causes the impedance presented by the primary winding to appear very small. The collector current in the horizontal output transistor therefore increases greatly and may cause breakdown of the transistor.
The high voltage transformer primary winding is coupled to the regulated B+ power supply and provides power via the transformer secondary windings for receiver circuits such as vertical deflection, signal processing and audio during normal receiver operation. During receiver start up, before the B+ power supply can provide the regulated B+ voltage, the previously described circuits may heavily load the secondary windings of the high voltage transformer. Because of this, horizontal output transistor collector current and hence the transformer primary winding current will increase in an attempt to supply the transformer secondaries with sufficient power. The transistor collector current may then increase to such an extent that damage to the transistor may occur.
A resistor may be placed in the output transistor conduction path to limit transistor collector current, but as the transistor also conducts deflection yoke current, the added resistance, which may be of the same order of magnitude as the resistance of the horizontal deflection coil, may degrate the deflection waveform linearity, resulting in raster distortion.